A film forming apparatus which implements a so-called ALD (Atomic Layer Deposition) method as a method of forming a thin film such as a silicon oxide film or the like on a substrate such as a semiconductor wafer or the like (hereinafter referred to as “wafer”) is known. As the apparatus for implementing the ALD method, an apparatus is known in which a plurality of wafers disposed on a rotary table within a vacuum container is revolved by the rotary table such that the wafers sequentially pass through a supply region of a raw material gas and a supply region of a reaction gas reacting with the raw material gas. In this apparatus, the raw material gas is supplied by a gas nozzle which extends in the radial direction of the rotary table and which has gas discharge holes formed along the longitudinal direction of the gas nozzle. A baffle plate is installed above the gas nozzle to increase the efficiency of adsorption of the raw material gas onto the wafers.
A nitrogen gas as a carrier gas is supplied together with the raw material gas from the gas nozzle. Due to the flow rate of the carrier gas, the discharge amount of the raw material gas varies along the longitudinal direction of the gas nozzle. For that reason, there is a need to set the flow rate of the carrier gas at a suitable value and to regulate the gas concentration in the longitudinal direction of the gas nozzle, thereby securing good in-plane uniformity. The control of in-plane uniformity of a film thickness through the regulation of the flow rate of the carrier gas is also performed when starting up the apparatus or when re-assembling the apparatus after maintenance. However, the apparatus mentioned above poses a problem in that the flow rate range of the carrier gas for the securement of high in-plane uniformity is narrow and the calibration of the flow rate of the carrier gas between apparatuses and before and after maintenance is difficult to perform, which makes regulation troublesome.